Stanford University physicists have achieved a major milestone in quantum mechanics, directly observing the quantum jumps of sound in a mechanical resonator for the first time. The breakthrough bridges a century-old gap in quantum physics research and paves the way for advanced quantum technologies built on acoustic waves rather than light or electricity.
The findings, published in the journal Science, mark the latest chapter in the exploration of quantum jumps—sudden, discrete transitions from one energy state to another. While these phenomena have been fundamental to quantum theory since the early decades of the 20th century, capturing them experimentally has proven exceptionally challenging depending on the physical medium involved.
Researchers first demonstrated quantum jumps in trapped atomic ions back in 1986. More than two decades later, in 2007, scientists successfully observed similar transitions in photons, the fundamental particles of light. Sound, however, remained a stubborn target due to the complex ways acoustic vibrations interact with their environments and the extreme difficulty of measuring them without destroying the fragile states in question.
A research team led by Amir Safavi-Naeini, an associate professor of applied physics in the Stanford School of Humanities and Sciences, has now overcome these hurdles by recording acoustic quantum jumps directly and in real time.
"What this study shows will allow us to move forward with developing new quantum technologies with sound," Safavi-Naeini said. "We have seen that vibrating objects can exhibit quantum behavior, which is the prerequisite for many of the operations needed by quantum computing and sensing."
Watching Sound Behave Quantum Mechanically
In the realm of classical physics, everyday experiences dictate how we understand sound and vibration. In daily life, mechanical vibrations appear to fade smoothly and continuously. A striking bell, for example, gradually grows quieter as its energy dissipates until the sound disappears entirely into the background.
At the quantum scale, however, the picture is fundamentally different. Just as light is quantized into discrete packets known as photons, sound has a quantum equivalent called the phonon, which represents the coordinated, collective motion of many atoms within a material.
Instead of fading continuously, a quantum mechanical resonator’s vibrational energy changes in distinct, abrupt steps. Earlier experiments across various laboratories had produced tantalizing indirect evidence that sound could undergo these transitions, but the Stanford team’s new study goes significantly further by directly tracking individual phonons as they make quantum jumps in real time.
A Microscopic Resonator With an Unusually Long Ring
To capture these elusive phenomena, the Stanford researchers utilized a tiny mechanical resonator manufactured using advanced semiconductor chip fabrication techniques. Because of its microscopic scale, many such resonators could potentially be integrated onto a single silicon chip, opening the door to complex architectures capable of carrying out sophisticated computational tasks.
A critical engineering feature of the device was its exceptionally long ringdown time. Acting much like a microscopic tuning fork, the resonator is capable of sustaining its vibrations for approximately two milliseconds. To put that persistence into perspective, if a normal-sized tuning fork possessed the same relative ability to sustain vibrations, it would continue ringing audibly for several hours.
That unusually long-lived vibration was vital to the success of the experiment. It provided the research team with a wide enough temporal window to collect hundreds of repeated measurements while the system was active. Those sequential readings allowed the scientists to precisely identify the exact moment when the mechanical vibration faded, marking the moment the sound energy jumped downward from an energy state of 1 to 0.
Measuring a Fragile Quantum State
Beyond fabricating the resonator itself, the experiment required the research team to solve a classic, long-standing problem in the field of quantum engineering: how to measure what is happening inside a delicate quantum system without destroying or disrupting the very state being observed.
Takuma Makihara and Erik Szakiel, co-first authors on the study, developed an innovative method for coupling the microscopic mechanical resonator directly to a superconducting qubit. A qubit is an electrical circuit capable of storing quantum information and processing it according to the laws of quantum mechanics. In this experimental setup, the qubit played a dual role, also serving as a sensitive electrical detector.
"We had to continually develop new processes to make this extremely long-lived, vibrating object and then integrate it with the qubit, which is our little electrical detector — without ruining either subsystem," said Makihara, who recently completed his doctoral studies at Stanford.
During the two milliseconds that the mechanical resonator vibrates, the superconducting qubit repeatedly checks its status, determining whether the phonon is in an energy state of 1 or of 0. By performing these measurements rapidly and repeatedly over the course of the vibration, the researchers can pinpoint precisely when the quantum jump occurs.
Toward Quantum Computing and Ultra-Sensitive Sensors
The Stanford researchers view their achievement as an early yet foundational step toward practical technologies that harness sound as a primary quantum platform.
One of the most promising potential applications lies in the realm of quantum error correction. While theoretical quantum computers hold the potential to solve extraordinarily complex calculations that remain entirely out of reach for conventional supercomputers, their underlying quantum states are notoriously fragile. Environmental disturbances can introduce errors long before a complex calculation reaches its completion.
In many proposed quantum computing architectures, a sudden quantum jump can serve as a reliable indicator that an error has occurred within the system. Because detecting those jumps has historically been difficult, the ability to monitor them reliably using sound could provide scientists with an important new diagnostic tool for identifying and correcting quantum errors on the fly.
Furthermore, the successful combination of the mechanical resonator and the superconducting qubit could evolve into a highly sensitive measurement platform. Safavi-Naeini’s research group is already collaborating with a team led by physicist Michael Roukes at the California Institute of Technology to explore whether the system can be adapted to detect and identify individual proteins inside biological cells.
Better Control of Sound
Beyond specialized quantum computing and sensing architectures, the advance may eventually yield benefits for more conventional electronic technologies. Sound plays an integral role in modern smartphones, wireless communication devices, and countless other everyday electronics, and achieving increasingly precise control over acoustic vibrations could pave the way for entirely new generations of commercial hardware.
According to Szakiel, a current doctoral student in Safavi-Naeini’s laboratory, the demonstration highlights a new level of command over mechanical waves.
"This shows we can have incredibly fine-tuned control of sound, which might mean that devices that use sound as a fundamental technology can get much better," Szakiel said.
Safavi-Naeini is affiliated with Stanford Q-FARM and Bio-X. Additional co-authors on the research include David Schuster, the Joan Reinhart Professor and professor of applied physics in the Stanford School of Humanities and Sciences; Shannon Harvey, a scientist with the SLAC National Accelerator Laboratory; Mihir Pendharkar, a physical research scientist at the Edward L. Ginzton Laboratory; former applied physics doctoral scholar Rachel Gruenke-Freudenstein; and Oliver Hitchcock, Matthew Maksymowych, and Kaveh Pezeshki, all doctoral scholars in applied physics at Stanford.
The research was made possible with financial and material support from Amazon Web Services Inc., the Air Force Office of Scientific Research, the Office of Naval Research, the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the U.S. Department of Defense. Both Safavi-Naeini and Schuster serve as Amazon Scholars.
